Pressure casting machine with pressure increase system

ABSTRACT

A pressure casting machine having a moulding piston slidable in a cylinder and a pressure-increase system having a stepped piston and serving to increase the pressure in the moulding cylinder towards the end of the working stroke of the moulding piston, the pressure line leading to the working chamber of the moulding cylinder being closed by means of a valve when the pressureincrease system is in operation, wherein the annular face of the pressure-increase piston is free from pressure and the large face of the said piston is actuated by a predetermined pressure through a feed line controlled by a valve.

United States Patent Piekenbrink et al.

[451 Sept. 26, 1972 [54] PRESSURE CASTING MACHINE WITH PRESSURE INCREASE SYSTEM Filed: May 26, 1970 Appl. No.: 40,574

Inventors:

Foreign Application Priority Data April 15, 1970 Germany ..P 20 17 951.5

References Cited UNITED STATES PATENTS 4/1953 Holder ..l64/315 Roles 164/120 Bachelier ..18/30 QM X Primary Examiner-R. Spencer Annear Att0rneyl-lolman, Glascock, Downing & Seebold [57] ABSTRACT A pressure casting machine having a moulding piston slidable in a cylinder and a pressure-increase system having a stepped piston and serving to increase the pressure in the moulding cylinder towards the end of the working stroke of the moulding piston, the pressure line leading to the working chamber of the moulding cylinder being closed by means of a valve when the pressure-increase system is in operation, wherein the annular face of the pressure-increase piston is free from pressure and the iarge face of the said piston is actuated by a predetermined pressure through a feed line controlled by a valve.

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PRESSURE CASTING MACHINE WITII PRESSURE INCREASE SYSTEM The invention relates to a pressure casting machine having a moulding piston slidable in a cylinder and a pressure-increase system having a stepped piston and serving to increase the pressure in the moulding cylinder towards the end of the working stroke of the moulding piston, the pressure line leading to the working chamber of the moulding cylinder being shut by means of a valve when the pressure increase system is in operation.

In conventional pressure-casting machines, the pressurized fluid is conveyed to the working chamber of the moulding cylinder through the piston of the pressure increase system. Within the latter piston, and in the path of flow of the pressurized fluid, there is located a blocking piston or non-return valve. In addition, the annular chamber in the cylinder of the pressure-increase system, and hence the annular surface of the pressure increase piston, is under a certain pressure. This is necessary inorder to prevent the pressure increase piston from moving as the moulding piston moves forward to fill the-mould. If such a counter-pressure were not provided, the pressure increase piston would be set in motion during the mould-filling operation, since the presurized fluid flows not only through the pressure increase piston but simultaneously onto the large rear surface of the said piston and because this latter pressure is greater than the pressure in the working chamber of the-moulding cylinder becauseof the drop in pressure within the pressure increase piston which occurs'as the fluid passes through the blocking piston or non-return valve.

Since it is important that once the moulding piston has completed its working stroke, pressure should be built up again very rapidly, it is necessary for the blocking piston or non-return valve to close immediately at the right time and that the-pressurized fluid which has hitherto served to maintain a counter-pressure should be able to flow rapidly out of the annular chamber in the pressure increase cylinder so as to reduce pressure. When it is desired to maintain a reduced counter-pressure, the pressurized fluid must be controlled or regulated by other means as it flows out of the annular chamber in the pressure-increase system. Because the full pressure from the source of pressurized fluid acts on the large surface of the piston, the degree of subsequent pressure build-up can only be achieved by influencing conditions in the annular chamber in this manner.

The rapid drainage of a large amount of pressurized fluid from the annular chamber'in the pressure-increase system poses, however, a considerable number of problems. If the counter-pressure in this chamber is required to be to some degree adjustable, additional problems are raised. The timing of the pressure buildup after the working stroke and the achieving of the correct amount of pressure is also dependent upon the blocking piston located in the pressure-increase piston directly affect the functioning of the machine and the quality of the products.

The aim of the invention is to overcome the existing disadvantages and difficulties and to construct a pressure casting m'achine in such a manner that it is possible for the pressure after a working stroke to be built up rapidly by means of a pressure-increase mechanism in a particularly favorable manner and to the desired value. The invention also seeks to achieve an efficient and advantageous arrangement of the means used.

Starting from a pressure casting machine of the type above described the invention provides for the annular surfaces of the pressure-increase piston to be free from pressure while the large surface of the said piston is acted upon by a pressure which may be previously adjusted through a feed line controlled by a valve.

The invention thus departs from the conventional principle of filling the annular chamber in the pressureincrease cylinder with a pressurized fluid in order to produce a counter-pressure and to empty it again at each working stroke to a certain degree. Working on the principle newly adopted by the invention, and hence eliminating the operation of filling the annular chamber with pressurized fluid, means that all the disadvantages associated therewith are also eliminated without any other problems arising to take their place.

In the machine of the invention, the large face of the pressure. increase piston can be acted upon at exactly the right moment with the amount of pressure required at a given moment, so that optimum functioning of the pressure-increase system and hence of all following elements is ensured. Whereas in conventional arrangements, the pressure increase system is acted upon by a substantial pressure, which has to be compensated in part by a counter-pressure, it is now sufficient to act upon only one face of the piston with a pressure which is correspondingly reduced. This may be effected using a suitable source of compressed fluid.

The invention further provides for the feed line of the pressure increase system to be connected to a pressure line through a pressure-regulating or pressure reducing valve.

In one particularly advantageous embodiment of the invention a pressure reservoir is connected to the feed line for the pressure-increase system. Thus when necessary a greater amount of pressurized fluid can be conveyed to the pressure-increase system very rapidly.'The reservoir is then previously charged with the desire final pressure.

Particularly in cases where a considerable static period is to be expected during the period of pressure build-up, during which time the pressure has to be kept constant, it is of advantage to operate the reservoir using a pressure setting or pressure reducing valve so that pressurized fluid may, when necessary, be drawn from the feed line.

The control valve for the feed line to the pressure-increase system is advantageously dependent upon the pressure in the working chamber of the moulding cylinder. The member actuating the valve may respond directly to a given pressure, or the valve may be actuated by means of a pressure switch which is preferably adjustable and which acts when a preset pressure is reached so as to energize an electrical member which actuates the valve.

The invention particularly provides for the shut-off valve to be arranged between the working chamber of the moulding cylinder an the high-pressure chamber of the pressure-increase system. The arrangement is very advantageously effected in such a manner that the shutoff valve also serves as a valve to produce a connection between the working chamber of the moulding cylinder and a high-pressure chamber of the pressure-increase system which is separate from it, so that the working chamber of the moulding cylinder and the high-pressure chamber of the pressure-increase system are connected when the pressure line to the working chamber of the'moulding cylinder is blocked and vice versa.

According to a further feature of the invention, the shut-off valve is actuated by a pressure produced in the high pressure chamber of the pressure-increase system. This makespossible a particularly rapid and reliable response on the part of the valve. In one particularly advantageous embodiment the shut-off valve is provided with a valve member which may be displaced by the pressure in the high-pressure chamber of the pressure-increase system, and which is provided with holes which are closed by the walls of the bore which receives it and which effect the connection between the working chamber of the moulding cylinder and the high-pressure chamber of the pressure increase system.

In order to return the valve member to the position in which it isolates the working chamber of the moulding cylinder from the high' pressure chamber of the pressure increase system, it is possible, for example, to provide a tappet which moves when a predetermined pressure level is reached in the chamber into which the moulding piston is retracted.

In another embodiment the invention provides for a part of the pressure increase piston itself to constitute a closure member which closes the pressure line leading to the working chamber of the moulding cylinder during operation of the pressure increase system.

In an arrangement of this kind, the pressure increase system and a valve member which closes the pressure line leading to the working. chamber of the moulding cylinder are no longer coupled hydraulically but combined in a single mechanical unit. This results in a particularly simple design involving few parts. There is no longer a special high-pressure chamber between the pressure increase piston and separate valve body. The working chamber in the moulding cylinder now becomes the high pressure chamber of the pressure increase system during operation of the said pressure increase system. The result is a reduction in the hydraulic system and in the overall size of the machine.

In one very advantageous embodiment the pressureincrease piston has a valve surface such as a shoulder, a conical surface or the like while a control sleeve is slidably arranged in a cylinder chamber located in front of the said valve surface, the said sleeve constituting a valve seating facing the valve surface of the pressure increase piston.

This results in more reliable closure of the line which conveys pressurized fluid to the working chamber of the moulding cylinder during the mould filling operation. The slidability of the control sleeve makes it possible, while maintaining the full closure effect, for the pressure increase system still to move forward, as may be necessary during the after-pressure operation because of the compressibility of the pressurized medium and in order to compensate for leakage losses.

Specifically the arrangement may be such that the pressure increase piston has a projection which engages with the control sleeve and carries on its front end a stop member for the control sleeve. This projection may take any one of a number of forms. Thus, it may be hollow and be provided, in the part adjacentthe valve surface of the pressure increase piston, with holes through which the pressurized fluid conveyed'to the working chamber of the mould cylinder is able to flow. It is'also possible to provide, between the projecting portion of the pressure increase piston and the control sleeve, passages for thepressurized fluid conveyed to the working chamber of the mould cylinder. The projection may then'take the form of a central rod which carries on its front end a perforated plate or spoke-like pins which constitute the stop member for the control sleeve. The diameter of the rod is so chosen in relation to the internal diameter of the control sleeve as to ensure that the diameter of the passage is sufficiently large.

Another possible method of closing the pressure line conveying fluid to the working chamber of the mould cylinder using the pressure increase piston consists in arranging the inlet to this line at a location on the cylinder which is covered by a part of the pressure increase piston during the latters working stroke. 7

Further details, features and advantages of the invention will become clear from the following description with reference to the accompanying drawings, in which:

FIG. 1 shows in cross-section the moulding cylinder and pressure increase system of a machine according to the invention the associated hydraulic elements'being shown in part in diagrammatic form;

FIG. 2 shows the moulding cylinder and the pressure increase system with the associated elements of another embodiment of the invention;

FIG. 3 is a variant shown in cross-section so as to correspond to the center portion of FIG. 1;.

FIG. 4 is a section along line IIIIII in FIG. 2;

FIG. 5 is a variant shown in section in a view corresponding to that of FIG. 4; and

FIG. 6 is a sectional view of a further embodiment of the pressure increase system; and

FIG. 7 is a side view of a pressure casting machine with a pressure increase system.

The pressure casting machine which is shown in part diagrammatically in FIG. 7 has a base frame G with a cross-piece T arranged thereon, and also a fixed clamping plate F1. The cross-piece T is connected to the fixed mould clamping plate F1 by means of four columns S on which the movable mould clamping plate F2 is slidable by means of an elbow lever arrangement K. On the right hand side of the machine as shown in FIG. 7, is located the pressure increase system M, the moulding cylinder PZ with moulding piston PK which presses the liquid metal into the mould, and also a reservoir arrangement SP.

The moulding piston 2 is slidable together with its piston rod inthe moulding cylinder 1. The cylinder chamber 3 which is shown in the drawing on the right of the moulding piston 2 is referred to as the working chamber and that to the left of the piston 2, viz. the chamber 4, is referred to as the retraction chamber. Pressurized fluid is conveyed to the latter from a source of pressurized fluid (not shown) such as a pump, through a valve 5 and line 6, when, after the casting process, the moulding piston 2 is to be returned to its initial position shown in the drawing.

In order to urge the moulding piston 2 forward in the other direction to effect the casting process, pressurized fluid is conveyed to the working chamber 3 of the moulding cylinder from the source of pressurized fluid through the valve 5,- a line 7 and a line 8. This is indicated in the drawing by means of arrows. A pressure reservoir 9, which may be a piston-type reservoir is actuated at a given moment by means of a controllable valve 10 and is also able to convey a large quantity of pressurized'fluid rapidly to the working chamber 3 of the moulding cylinder through the line 8.

A pressure-increase cylinder 11 with the piston 12 slidable therein, is arranged transversely to the axis of the moulding cylinder 1. The piston may be actuated on its large piston face 13, which is shown at the bottom in the drawing, by means of a feed line 15 provided with a switching valve 14, with fluid at a predetermined pressure. The smaller pistonface l6, which'is shown at the top in the drawing, has opposite it the high pressure chamber 17 of the pressure-increase system. The annular chamber 18' is open to the atmosphere through a ventilating or waste oil return line 19. The annular face 20 of .the pressure-increase piston 12 is thus permanently free from pressure. At thev end' of the pressure-increase cylinder 11 shown at the top in the drawing, there is located a tubular valve member 22 which slides into a bore 21 and which is provided, adjacent its front end, with holes 23. This valve 22 fulfils a dual function. In the position shown in the drawing, it opens the passage between the line 8 and the working chamber of the' moulding cylinder 3 and at the same time cuts off the said working chamber from the high-pressure chamber 17 of the pressure-increase system. In its extended position the valve member 22 abuts againstthe seating 24 and thus closes the connection between the line 8 and the working chamber of the moulding cylinder, while the high pressure chamber 17 of thepressure-increase system is now connected to the working chamber f the moulding cylinder through the holes 23 in the valve member 22. a r

A line 25 leads from theworking chamber of the moulding cylinder to a pressure switch 26 which may be set to a particular level and which actuates the switching valve 14 so as to cause thetvalve to move from the position shown in the drawing, in which it is held by a'spring and in which the chamber 27 of the pressure-increase cylinder isconnected to the vessel containing the pressurized fluid, and into the second of its two positions, in which the feed line 15 is connected to the chamber 27. A pressure reservoir 28 is connected to this feed line 15. The feed line 15 and the pressure reservoir 28 are connected, through an ad-.

, regulating valve 29 ensures that the reservoir 28 is kept constantly at the preset pressure which is available directly, when the feed line 15 is connected, by means of the valve 14, to actuate the surface 13 of the pressure-increase piston 12.

A tappet 31 moves'the valve member 22 back out of the position in which it blocks the line 8 and into the position shown in the drawing. The tappet 31 is located in a servo piston 33 which is able to slide in a cylinder 32. Pressurized fluid may be conveyed to the cylinder 32 from the line 6 through a line 34 and a valve 35 which may be'adjusted to a preset value. Parallel to this valve 35 there is a line 36 with a non-return valve37.

A working cycle is as follows. Starting from the position shown in the drawing for the various parts, the valve 5 is first moved so that the line 7 is connected to the source of pressurized fluid and the line 6 to the tank. Thus, the pressurized fluid flows into the working chamber 30f the moulding cylinder through the lines 7 and 8 so'that the moulding piston 2 begins to move. In this first stage of the working cycle, the moulding piston 2 only moves slowly, until the filling. aperture for the machine moulding chamber, which is not shown in the drawing, is closed. Actuation of -a switch or other control means then causes the valve 10 to 'open and also opens the pressure reservoir 9. As aresult of the extra fluid which now flows through the line 8 into the working chamber of the moulding cylinder 3, the mouldingpiston 2 isurged forward at greater speed.

i This is the second stage, in which the hollow area of the pressure casting mould is filled with liquid metal.

Towards the end of the stroke of the moulding piston, the pressure in the working chamber of the mouldingcylinder rises because the mould is now full.

"As a result, the pressure switch 26 now responds and moves the v valve 14 into the openposition, so that the large facel3'of the pressure-increase piston 12 is actuatedby the predetermined pressurewhich reigns in the reservoir 28. At the same time,-the.corresponding excess pressure builds up in the high-pressure chamber 17 ofthe pressure-increase system. The valve 22 is thus pushed out and urged against its seating 24. It thus connects the high'pressure chamber 17 of-the pressure increase systemto the'working chamber 3' of the mould- .ing cylinder, while the line 8 is blocked. This dynamic portion of the pressure build-up stage is then followed this being chosen as the optimum for given operating conditions. In orderto ensure that the moulding piston is returned after completion of the casting operation, the valve 5 is switched over, sothat the line 7 is now connected to the tank while'the line 6, and thus the retraction chamber 4, is filled with pressurized'fluid. This causes the moulding piston 2'to move to the right and push the fluid present in the chamber 3 through the holes 23 in the valve member 22 into the chamber 17 of the pressure-increase cylinder ll,-so that the pressureincrease piston 12 also moves back toinitial position.

The pressure drop in the working chamber 30f the moulding cylinder has also caused the valve-=14 to move back to its illustrated rest position, in which the chamber 27 of thepressure-increase cylinder 11 is open to the tank, so that the fluid which was urged into this chamber by the return movement .ofthe pressureincrease piston 12 can now drain off freely.

Since pressure in the chamber 4 of the moulding cylinder 1 rises at the end of the return movements of the piston 2 and piston 12, the appropriately preset valve 35 opens so that the piston 33 is actuated and the tappet 31 acts to return the valve member 22 to the rest position shown in the drawing. During the next working cycle, the piston-33 is actuated through the passage 38 at its other side by the working pressure and thus returns to initial position. The displaced fluid then flows through the lines .34, 36, the non-return valve 37, the line 6 and the valve back to the tank.

The other components of the pressure casting machine, including the control system, which may make use of a sequence control, may be of conventional design so'that there is no need to describe them here.-

The other parts of the pressure casting machine, including the control system in which, for example, it is possible to use elements of a follow-up control, may be of conventional type and design, so that there is no need to discuss these in more detail.

In the embodiment shown in FIG. 2, the moulding piston 42 slides together with its piston rod'in a moulding cylinder 41.,The cylinder chamber 43 shown to the right ofthe moulding piston42 will be referred to as the working chamber and that to the left of the moulding piston 42, viz. chamber 44, as the retraction chamber. Pressurized fluid is conveyed to the retraction chamber from a source of pressurized fluid (not shown), such as a pump, from a line 46 through a valve 45 and line 46a, when, after the moulding operation it is desired to retract the moulding piston 42 from the right hand initial position shown in the drawing.

In order to advance the moulding piston 42 in the other direction in order to carry out the moulding operation, pressurized fluid is conveyed to the working chamber 43 of the moulding cylinder from the source of fluid through the valve 45 and a line 47. A pistontype pressure reservoir 49 may at a given moment be actuated by means of a controllable valve 50 in order rapidly to convey fluid through a line 48 to the working chamber 43 of the moulding cylinder in a large quantiv A pressure increase cylinder 51 is arranged coaxially to the moulding cylinder 41 and a pressure increase cylinder 52 is slidable therein. This, piston 52 is a stepped piston having a portion 52a of large diameter and a shouldered portion 52b of smaller diameter. The pressure increase piston may be actuated by pressurized fluid at a predetermined pressure through a feed line 55 with a switching valve 54, on 'the large piston face 53 which is shown on the right in the drawing. The annular chamber 58 located in front of the large portion 52a of the pressure increase piston 52 admits atmospheric air through a venting or leak oil return line 59. The annular face 60 of the pressure increase piston 52 is thus continuously relieved of load.

The front end of the shouldered portion 52b of the pressure-increase piston has a conically shaped valve face 56. A control sleeve 62 is slidablymounted in the portion 57 of the cylinder 42 located between the said valve face 56 and the'moulding piston 42. This sleeve has on its end adjacent the portion 52b of the pressure increase piston a valve seating 64 against which the valve face 56 may abut against the part 52b of the pressure increase piston.

The movement of the control sleeve 62 towards the pressure-increase piston 52 is limited by a stop face 61 in the cylinder. The arrangement is, moreover, such that the control sleeve 62 encloses a hollow projection 71 on the pressure increase piston, the said projection 71 having a collar 72 which is locatedon the front end of the piston and constitutes a stop member for the control sleeve 62. The part of the projection 71 which is located adjacent the valve face 56 of the pressure increase piston has passages 63 for the pressurized fluid leaving the lines 47 and 48 and entering the working chamber 43 of the moulding cylinder.

From the working chamber of the moulding cylinder 43 a line 65 leads to a control member 66 which may be adjusted to a certain hydraulic pressure and acts upon the connecting valve 54 so as to move this out of the position shown in the drawing, in which it is held by a spring and in which the working chamber 67 of the pressure increase cylinder 51 is connected to the machine pressure reservoir, into the second possible position in which the feed line 55 is connected via the connecting line 55a to the pressure increase chamber 67. A pressure reservoir 68 is connected to the line 55 and both the line 55 and the pressure reservoir 68 are connected via a controllable pressure setting or pressure reducing valve 69 to a supply line 70, through which pressurized fluid may be conveyed from a source of fluid not shown specifically in thedrawing and which may be a pump. This may be identical with the source of pressurized fluid to which the valve 45 is connected. The pressure setting valve 69 ensures that the reservoir 68 is maintained at a constant pressure which serves, when the line 55 is connected through the valve 54, directly to actuate the large face 53 of the pressure increase piston 52.

A branch line 73 leading from the line 46 and containing a valve 74 enables the reservoir 49 to 'be kept constantly full.

A working cycle is as follows: Starting, for example, from the position of the various components shown in FIG. 2, the valve 45 is first moved so that the line 47 is connected to the source of pressurized fluid and the line 46a with the tank which is not shown specifically in the drawing. This causes the pressurized fluid to be conveyed to the working chamber 43 of the moulding cylinder through the line 47, passages 63 and the hollow chamber in the projection 71, so that the moulding piston commences a stroke. During this first stage of the working cycle the moulding piston only moves slowly until the inlet aperture to the moulding chamber of the machine (not shown) is closed. Actuation of a switch or other release mechanism then causes the valve 50 to be opened and hence a connection to the pressure reservoir 49 to be established. The extra amount of pressurized fluid flowing through the line 48 pressure casting mould is rapidly filled with molten metal.

As the piston nears the end of its stroke, the pressure in the working chamber 43 of the moulding cylinder increases because the mould is now full. As a result the control member 66 is actuated and brings the valve 54 into open position, so that the large surface 53 of the pressure increase piston 52 is acted upon, through the connecting line 53a, by the preset pressure of the fluid contained in the reservoir 68. The pressure increase piston 52 is thus caused to move forward and the valve face 56 on the part 52b of the piston moves into abutment against the seating 64 on the control sleeve 62. This shuts off the lines 47 and 48 so that they are no longer connected to the working chamber 43 of the moulding cylinder. The control sleeve 62 remains in position within the cylinder portion 57 until the valve face 56 reaches the seating 64, due to the pressure acting on the ends of the piston. It is possible also to arrange rings on the periphery in such a' manner that these rings abut against the wall of the cylinder 57 and also act to hold the control sleeve 62 stationary as a result of friction. From the moment of contact with the pressure increase piston onwards, the control sleeve is however able to follow a further forward movement of the pressure increasepiston. V 1

Pressure in the working chamber 43 of the moulding cylinder is now high as as result of the action of the increase system and sufficient for the after-pressure stage. This pressure remains effective for @a certain time. If the amount of pressurized fluid available in the pressure reservoir-68 is not sufficient on its own, further quantities of pressurized fluid are immediately conveyed at the correct pressure through the valve 69.

In order to retract the moulding piston 42 when the moulding operation has been completed, the valve 45 is switched over so that the line 47 is now connected to the tank, while the line 46a and hence the retraction chamber 44 is supplied with fluid. This causes the moulding piston 42 to move towards the right (FIG. 2), pushing the fluid contained in the chamber 43 through the passages 63 in the projection 71 of the pressure increase piston 52 and out through the line 47. At the same time the pressure increase piston itself is pushed back into its initial position which is shown in FIG. 1. As it does so, the collar 72 on the projection 71 draws the control sleeve 62 with it until the latter abuts on the stop face 61 in the cylinder and movement ceases. The drop in pressure in the working chamber of the moulding cylinder'also causes the valve 54 to return to the rest position shown, so that the chamber 67 in the pressure increase cylinder 51 is connected to the tank through the line 55a while the fluid acted upon by the pressure increase piston 52 during the return movement and displaced thereby is able to flow freely out of the said chamber.

In the variant shown in FIGS. 3 and 4, the shouldered portion 82b of the pressure increase piston 82 is provided with a rod-like extension 81, the free end of which carries a star formed by three radially extending spokes 83 (FIG. 4) and acting as a stop member for the control sleeve 62, so that it in fact has the same function as the collar 72 in the embodiment shown in FIG. 2. This star may be replaced, for example, by a disc 84 having apertures 85, such as is shown in FIG. 5. FIG. 3 shows that in this case the pressurized fluid flowing through the line 47, 48 passes through the passages 86 provided between the extension 81 and the internal surface of the control sleeve 62 and into the working chamber 43 of the moulding cylinder. In FIGS. 2 and 4, like parts are designated by like numerals to those in FIG. 2.

The same is also true for FIG. 6 which shows a further embodiment. Here the shouldered portion 92b of the pressure increase piston 92 covers the inlet apertures 93, 94 in the lines 47, 48 directly during its working stroke as it advances in the cylindrical seating 95, thereby closing off the working chamber 43 of the moulding cylinder.

We claim:

1. A pressure casting apparatus comprising a moulding cylinder, a moulding piston slidably disposed at least partially within said moulding cylinder thereby defining a working chamber and a retraction chamber within said moulding cylinder, means for supplying fluid under pressure to said working chamber to thereby actuate said moulding piston in its working stroke; and pressure-increase means for increasing the pressure in the working chamber towards the end of said working stroke responsive to a predetermined moulding pressure, said pressure-increase means comprising a pressure-increase cylinder, a pressure-increase piston slidably disposed within said pressure-in crease cylinder, said pressure-increase piston being stepped and having a large face, a small face, and an annular face, said large face thereby defining a pressure chamber, said small face thereby defining a high pressure chamber and said annular face thereby defining an annular chamber, means for maintaining said annular chamber at atmospheric pressure, separate means for supplying fluid pressure to said pressure chamber responsive to said predeterminedv moulding pressure, means for adjusting said pressure supplied to said pressure chamber to provide a predetermined final moulding pressure, and closure valve means for closing said means for supplying pressure to said working chamber when said pressure-increase means is in operation.

2. A pressure casting machine as claimed in claim 1, wherein said separate means for supplying pressure to the pressure-increase means comprises a pressurized fluid feed line and a pressure-regulating or pressurereducing valve.

3. A pressure casting machine as claimed in claim 2, wherein a pressure reservoir is connected to the feed line.

4. A pressure casting machine as claimed in claim 1, wherein said separate means for supplying pressure further comprises a switching valve responsive to the pressure in the working chamber of the moulding cylinder.

5. A pressure casting machine as claimed in claim 1, wherein the closure valve means also serves as a valve to connect the working chamber and said high-pressure chamber when the pressure line to the working chamber is blocked and vice versa.

6. A pressure casting machine as claimed in claim 1, wherein the closure valve means is operable by the pressure in the high pressure chamber of the pressureincrease means.

7. A pressure casting machine as claimed in claim 5, wherein the closure valve means has a valve member which is actuated by the pressure in the high pressure chamber of the pressure-increase means and has holes which are closed by the walls of the bore which receives it and which effect the connection between the working chamber of the moulding cylinder and the high-pressure chamber of the pressure increase means.

8. A pressure casting machine as claimed in claim 7, further comprising a tappet which moves when a predetermined pressure level is reached in the chamber into which the moulding piston is retracted to return the vale member to the position in which it isolates the working chamber of the moulding cylinder from the high-pressure chamber of the pressure-increase means.

9. A pressure'casting machine as claimed in claim 1, wherein the pressure increase piston has a valve surface and a control sleeve is slidably arranged in a cylinder chamber located in front of the said valve surface, the

said sleeve constituting a valve seating facing the valve surface of the pressure increase piston.

10. A pressure casting machine as claimed in claim 9, wherein the pressure increase piston has a projection which engages with the control sleeve and carries on its front end a stop member for the control sleeve.

11. A pressure casting machine as claimed in claim 10, wherein the projection is hollow and in the part adjacent the valve surface of the pressure increase piston has holes for the flow of pressurized fluid conveyed to the working chamber of the mould cylinder.

12. A pressure casting machine as claimed in claim 1 l wherein passages for the pressurized fluid conveyed to the working chamber of the mould cylinder are provided between said projection and the control sleeve. 

1. A pressure casting apparatus comprising a moulding cylinder, a moulding piston slidably disposed at least partially within said moulding cylinder thereby defining a working chamber and a retraction chamber within said moulding cylinder, means for supplying fluid under pressure to said working chamber to thereby actuate said moulding piston in its working stroke; and pressureincrease means for increasing the pressure in the working chamber towards the end of said working stroke responsive to a predetermined moulding pressure, said pressure-increase means comprising a pressure-increase cylinder, a pressure-increase piston slidably disposed within said pressure-increase cylinder, said pressure-increase piston being stepped and having a large face, a small face, and an annular face, said large face thereby defining a pressure chamber, said small face thereby defining a high pressure chamber and said annular face thereby defining an annular chamber, means for maintaining said annular chamber at atmospheric pressure, separate means for supplying fluid pressure to said pressure chamber responsive to said predetermined moulding pressure, means for adjusting said pressure supplied to said pressure chamber to provide a predetermined final moulding pressure, and closure valve means for closing said means for supplying pressure to said working chamber when said pressureincrease means is in operation.
 2. A pressure casting machine as claimed in claim 1, wherein said separate means for supplying pressure to the pressure-increase means comprises a pressurized fluid feed line and a pressure-regulating or pressure-reducing valve.
 3. A pressure casting machine as claimed in claim 2, wherein a pressure reservoir is connected to the feed line.
 4. A pressure casting machine as claimed in claim 1, wherein said separate means for supplying pressure further comprises a switching valve responsive to the pressure in the working chamber of the moulding cylinder.
 5. A pressure casting machine as claimed in claim 1, wherein the closure valve means also serves as a valve to connect the working chamber and said high-pressure chamber when the pressure line to the working chamber is blocked and vice versa.
 6. A pressure casting machine as claimed in claim 1, wherein the closure valve means is operable by the pressure in the high pressure chamber of the pressure-increase means.
 7. A pressure casting machine as claimed in claim 5, wherein the closure valve means has a valve member which is actuated by the pressure in the high pressure chamber of the pressure-increase means and has holes which are closed by the walls of the bore which receives it and which effect the connection between the working chamber of the moulding cylinder and the high-pressure chamber of the pressure increase means.
 8. A pressure casting machine as claimed in claim 7, further comprising a tappet which moves when a predetermined pressure level is reached in the chamber into which the moulding piston is retracted to return the vale member to the position in which it isolates the working chamber of the moulding cylinder from the high-pressure chamber of the pressure-increase means.
 9. A pressure casting machine as claimed in claim 1, wherein the pressure increase piston has a valve surface and a control sleeve is slidably arranged in a cylinder chamber located in front of the said valve surface, the said sleeve constituting a valve seating facing the valve surface of the pressure increase piston.
 10. A pressure casting machine as claimed in claim 9, wherein the pressure increase piston has a projection which engages with the control sleeve and carries on its front end a stop member for the control sleeve.
 11. A pressure casTing machine as claimed in claim 10, wherein the projection is hollow and in the part adjacent the valve surface of the pressure increase piston has holes for the flow of pressurized fluid conveyed to the working chamber of the mould cylinder.
 12. A pressure casting machine as claimed in claim 11, wherein passages for the pressurized fluid conveyed to the working chamber of the mould cylinder are provided between said projection and the control sleeve. 